1. Field of the Invention
This invention relates to a reflection-type photocathode (i.e. photoelectric surface), and a photomultiplier.
2. Related Background Art
Reflection-type photocathodes using nickel (Ni), etc. as the substrates are known in the art disclosed in a first literature, U.S. Pat. No. 4,160,185, a second literature, Japanese Patent Laid-Open Publication No. 87274/1974 and a third literature, Japanese Patent Publication No. 47665/1977.
The first literature discloses the art in which an aluminium oxide (Al.sub.3 O.sub.3) layer is formed on a Ni substrate, and antimony (Sb) is deposited on the Al.sub.2 O.sub.3 layer and is activated by alkali metals.
The Al.sub.2 O.sub.3 layer is provided for the prevention of the alloying of the Ni and Sb.
The second literature discloses the art in which a surface of an Al substrate (or a substrate having Al applied to a surface of a base) is oxidized to form an Al.sub.2 O.sub.3 layer, and a reflection-type photocathode containing Sb and alkali metals is formed. The base for Al to be applied to is exemplified by tantalum (Ta).
In the third literature as well, a surface of an Al substrate is oxidized to form an Al.sub.2 O.sub.3 layer, and a photocathode containing Sb activated by alkali metals is formed.
As described above, each of the conventional reflection-type photocathodes has the Al.sub.2 O.sub.3 layer below the activated Sb film which is a photosensitive layer. Therefore, their fabrication process essentially includes the step of oxidizing Al.
Photomultipliers are used for the photometry of feeble light, and are effective especially at a limit where light to be detected is measured by counting photons. Accordingly, the sensitivity improvement by even some percentage is significant, and the process control is very difficult.
A restrictive condition that the Al.sub.2 O.sub.3 layer is necessary not only lowers yields of their fabrication, but also makes it difficult to realize a stable sensitivity. Depending on characteristics of the Al.sub.2 O.sub.3 layer, the reflection-type photocathodes adversely have various sensitivities.